The present invention relates generally to an over-current protection method and circuit, and more particularly to a method and circuit for disconnecting power to a motor for a hermetic compressor upon detecting excess current.
Refrigeration systems, such as residential refrigerators, use electric motor powered hermetic compressors which compress the system refrigerant according to principles well known in the art. Under certain conditions, the compressor motor in a system can enter a fault mode wherein the power lines to the compressor input terminals carry excessively high current. This high current condition may result in a phenomenon commonly referred to as xe2x80x9cterminal ventingxe2x80x9d.
Terminal venting is generally characterized by a separation of the metallic compressor input terminal pins from the surrounding insulating material in which the pins are mounted. This can occur if excessively high current is supplied to the terminals for sufficient time to destroy the glass insulating seal. The problem is exacerbated by the different thermal expansion coefficients of the pins and the insulating material thereby causing destructive tensile stresses in the glass. The end result of such a failure is damage to the hermetic seal of the compressor terminal and, in some situations, the uncontrolled release of refrigerant gas.
Many compressor manufacturers incorporate mechanical safeguards into their compressor designs to reduce the likelihood and/or the effects of terminal venting. Some conventional compressors employ robust insulating materials with high temperature breakdown characteristics. Other compressors include covers which enclose the compressor terminals.
Conventional fuse-based interrupt circuits for similar applications do not adequately prevent terminal venting because such circuits are typically triggered by a prolonged presence of current levels substantially lower than the current levels associated with terminal venting. For example, when the compressor rotor becomes locked, the compressor motor draws high current (commonly referred to as xe2x80x9clocked rotor currentxe2x80x9d) such as 20 amps, for example, but not nearly as high as the current associated with terminal venting, which is typically in excess of 50 amps. Conventional interrupt circuits interrupt power to the compressor to protect the motor coils when the current draw of the compressor motor is in the range of locked rotor currents, and is sustained for a sufficiently long period of time. While the exceptionally high current associated with terminal venting would typically trigger a conventional interrupt circuit, the relatively slow response time of such circuits requires a prolonged application of this high current. Thus, damage to the compressor terminals may occur long before a conventional interrupt circuit is triggered.
It has been determined that if the temperature differential between the pin and glass exceeds a given threshold for a particular terminal, the resulting tensile stresses in the glass will cause failure of the pin-to-glass seal and result in terminal venting. In accordance with the method of the present invention and the particular exemplary circuit implementation shown, the current flowing through the terminal is detected. If the detected current exceeds a threshold level that, unless substantially immediately terminated, will cause the pin/glass temperature differential to rapidly exceed a threshold level resulting in glass stresses that will cause the pin-to-glass failure and terminal venting, power through the terminal is immediately terminated. The threshold current level is much higher than locked rotor current for the compressor motor, preferably at least two times the locked rotor current. It has been found that once the pin current exceeds a given threshold for a particular terminal, that even if the current rise is no higher, the pin and glass temperatures continue to rise and the pin/glass temperature differential where failure of the pin-to-glass seal occurs will rapidly be reached. Therefore, the threshold current selected for a particular terminal must be lower than that which correlates to simultaneous pin and glass temperatures at the failure level.
The present invention can be implemented by an exemplary protection circuit connected in series between the power lines and terminal of the compressor which detects the presence of a motor fault or other over-current condition and disconnects power to the terminal to prevent terminal venting due to this condition. The circuit generally includes a line-connected power supply for powering the circuit, a current sensor for sensing the current drawn by the compressor motor, and a control circuit for disconnecting power to the motor when a fault is detected. The circuit may include an audible or visual alarm to indicate the presence of a fault. Additionally, since the present protection circuit is connected in-line with the power connections to the compressor and external of the compressor housing, existing compressors may readily be retrofitted to obtain the protection against terminal venting provided by the present invention.
The method and circuit of the present invention protect the compressor terminals, as opposed to the motor coils, by quickly disconnecting power to the compressor, but only upon detection of exceptionally high current levels. This high threshold permits simultaneous operation of conventional interrupt circuits and prevents xe2x80x9cnuisance triggeringxe2x80x9d as a result of the large current demands at motor start-up or current noise spikes that may occur during operation. While the current threshold of the present protection circuit is quite high relative to the locked rotor current, damage to the compressor terminals is nonetheless prevented because the response time of the circuit is substantially faster than conventional interrupt circuits. For example, current is terminated within 20 milliseconds of detecting the preset current threshold. Thus, the exceptionally high current is removed from the compressor terminals before the temperature of the terminal pin causes damage to the pin-to-glass seal.